This invention relates to determining the invasion of drilling fluid into a core sample taken from a borehole.
When a well is drilled into a permeable formation a portion of the drilling fluid enters the formation and displaces the connate fluids, both brine and hydrocarbons, away from the borehole. It is important to know the depth of invasion, since all logging tools have some degree of sensitivity to the invaded zone. A core sample taken at depth will also experience this invasion. It is standard practice in the industry to analyze core samples to determine the depth of invasion into the core. It is also important to determine the depth of invasion into the core to know what portion of the core has been unaltered by the drilling fluid and therefore is representative of the unaltered formation.
Prior art workers have added tritium to the drilling fluid to determine the invasion of the drilling fluid into the core sample. In this method a core sample is cored from the borehole. A selection of the samples is cut from this core sample at increasingly radial distances from the center. Each of the cut samples is crushed, and the water in that sample is removed. The water from each sample is measured for approximately twenty-four hours with a Geiger counter to determine the radioactivity in that sample. A profile of the tritium invasion into the core sample is then plotted as a indication of the invasion of the drilling fluid into the formation. However, it has been found that this method provides less than desirable results, is time-consuming, has a large degree of statistical uncertainty, requires the handling of radioactive materials at the borehole and does not provide a cross-sectional view of the invasion.
Therefore, it is an object of the present invention to provide a method of determining the depth of invasion of the drilling fluid into a core that overcomes disadvantages and inaccuracies of the prior art.